Numerous instruments are employed in an airborne mode to carry out a wide variety of earth survey activities. Often, such surveys are conducted from relatively slow flying aircraft. By way of example, a camera may be mounted in a survey aircraft such that it is stabilized for roll, pitch and yaw whereby there is no relative motion between the camera and the ground except for velocity. If the film is advanced at the speed of the aircraft, a very good continuous photograph of the scene below may be obtained; however, there is no provision for sensing and compensating for transient dv/dt which may be brought about by wind gusts or other perturbations, and, as a result, the actual photograph may be randomly distorted longitudinally.
As another and related example, an airborne survey conducted to secure geological information is fairly typically carried out from an aircraft flying a carefully defined grid pattern at as constant a ground velocity as may be reasonably possible and which often may not be much in excess of 100 miles per hour. The fundamental reason for flying such closely controlled paths is to preserve the ability to later find, on the ground, any sort of geological anomoly which may be of investigative interest. Aircraft systems employed for such purposes as exploration for oil, gas, and other minerals usually carry sophisticated inertial navigation equipment which may be augmented by beacons temporarily placed at precisely known points within the area of operation. As information from whatever sensing instruments are in use is recorded (typically on a high density tape machine), navigational information is also recorded to provide a record of the aircraft's instantaneous position through the mission.
As is well known in the art, one aspect of preparing overlays to standard maps in order to relate any sensed anomoly to a precise ground position on the map is the extent to which brief perturbations of the aircraft ground velocity, such as may be caused by wind gusts, distort the aircraft's apparent instantaneous position with respect to its actual instantaneous position. If the apparent instantaneous aircraft position is recorded often enough, along with instantaneous acceleration/deceleration information, such distortions can be computer processed for substantial elimination. However, it is apparent that the computer processing power and time required can be significantly reduced if the aircraft's ground velocity could be closely maintained to the setpoint; i.e., to the nominal ground velocity predetermined for use during an operative pass. Additionally, it may be generally stated that the smaller the navigational error which must be corrected by computer processing, the smaller the residual error remaining after such processing will usually be.
Apparatus is often included in such survey aircraft to provide either or both of repetitive still photographs and video recordings of the ground scene below to provide further information to aid in subsequently finding and investigating discovered anomolies. As noted above, the more closely the picture taking mechanism (which is often actuated in fixed time increments) can be synchronized to the actual aircraft ground velocity, the more accurate will be the actual position depicted in the views recorded. For this reason also, the achievement of close control of the survey aircraft ground velocity is highly beneficial.
A survey aircraft can be guided along its predetermined flight path manually or under the control of an autopilot. In the prior art, to assist in holding the ground velocity steady, the autopilot may include an accelerometer, or the aircraft may have an independent accelerometer or the pilot may simply react to sensed or observed acceleration/deceleration. The output signal from either source of instantaneous aircraft acceleration/deceleration information is typically employed to automatically adjust engine speed to compensate for wind gusts and like perturbations to the aircraft ground velocity, or the pilot can adjust the engine speed manually; however, those skilled in the art are very aware that the compensation achieved by controlling engine speed is unsatisfactorily slow to react because of the high inertia of the engine and, indeed of the entire "system". That is, if acceleration caused by a gust from the rear is to be counteracted by momentarily slowing the engine, not only must the engine inertia be overcome, but also the aircraft forward inertia. Conversely, if deceleration caused by a gust from the front is to be counteracted by momentarily increasing engine speed, both the engine and aircraft inertia must be overcome. If the aircraft is under manual control, the pilot "inertia" is a manifest further source of inaccurate and too slow response.
There are similar conditions in which air velocity, rather than ground velocity, must be maintained as steady as possible. For example, during interaction between two aircraft, such as encountered in airborne refueling, it is very desirable to hold the two aircraft as close as possible to the predetermined optimum spacing and relative positioning.
It will therefore be readily appreciated by those skilled in the art that it would be highly desireable to provide an automatic, low inertia means for closely regulating the ground or air velocity of an aircraft carrying out a mission at a predetermined nominal velocity.